/* Copyright 2017 The TensorFlow Authors. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
==============================================================================*/
#include "tensorflow/core/kernels/data/prefetch_dataset_op.h"

#include <algorithm>
#include <deque>
#include <limits>
#include <string>

#include "tensorflow/core/data/dataset_utils.h"
#include "tensorflow/core/data/name_utils.h"
#include "tensorflow/core/data/stats_utils.h"
#include "tensorflow/core/framework/dataset.h"
#include "tensorflow/core/framework/metrics.h"
#include "tensorflow/core/framework/model.h"
#include "tensorflow/core/framework/partial_tensor_shape.h"
#include "tensorflow/core/framework/stats_aggregator.h"
#include "tensorflow/core/framework/tensor.h"
#include "tensorflow/core/lib/gtl/cleanup.h"
#include "tensorflow/core/lib/strings/str_util.h"
#include "tensorflow/core/lib/strings/stringprintf.h"
#include "tensorflow/core/platform/status.h"
#include "tensorflow/core/platform/stringprintf.h"
#include "tensorflow/core/profiler/lib/traceme.h"
#include "tensorflow/core/profiler/lib/traceme_encode.h"
#include "tensorflow/core/protobuf/error_codes.pb.h"

namespace tensorflow {
namespace data {

// See documentation in ../../ops/dataset_ops.cc for a high-level
// description of the following op.

/* static */ constexpr const char* const PrefetchDatasetOp::kDatasetType;
/* static */ constexpr const char* const PrefetchDatasetOp::kInputDataset;
/* static */ constexpr const char* const PrefetchDatasetOp::kBufferSize;
/* static */ constexpr const char* const PrefetchDatasetOp::kOutputTypes;
/* static */ constexpr const char* const PrefetchDatasetOp::kOutputShapes;
/* static */ constexpr const char* const PrefetchDatasetOp::kSlackPeriod;
/* static */ constexpr const char* const PrefetchDatasetOp::kLegacyAutotune;
/* static */ constexpr const char* const PrefetchDatasetOp::kBufferSizeMin;

namespace {

// Determines the fraction of slack time by which to delay prefetching of data.
constexpr double kSleepFactor = 0.2;
constexpr char kBuffer[] = "buffer";
constexpr char kStatus[] = "status";
constexpr char kSizeSuffix[] = ".size";
constexpr char kCodeSuffix[] = ".code";
constexpr char kErrorMessageSuffix[] = ".error_message";

}  // namespace

class PrefetchDatasetOp::Dataset : public DatasetBase {
 public:
  Dataset(OpKernelContext* ctx, const DatasetBase* input, int64_t buffer_size,
          int64_t slack_period, bool legacy_autotune, int64_t buffer_size_min)
      : DatasetBase(DatasetContext(ctx)),
        input_(input),
        buffer_size_(buffer_size),
        slack_period_(slack_period),
        legacy_autotune_(legacy_autotune),
        buffer_size_min_(buffer_size_min) {
    input_->Ref();
  }

  ~Dataset() override { input_->Unref(); }

  std::unique_ptr<IteratorBase> MakeIteratorInternal(
      const string& prefix) const override {
    return std::make_unique<Iterator>(Iterator::Params{
        this, name_utils::IteratorPrefix(kDatasetType, prefix)});
  }

  const DataTypeVector& output_dtypes() const override {
    return input_->output_dtypes();
  }

  const std::vector<PartialTensorShape>& output_shapes() const override {
    return input_->output_shapes();
  }

  string DebugString() const override {
    return name_utils::DatasetDebugString(kDatasetType);
  }

  int64_t CardinalityInternal(CardinalityOptions options) const override {
    return input_->Cardinality(options);
  }

  Status InputDatasets(std::vector<const DatasetBase*>* inputs) const override {
    inputs->push_back(input_);
    return OkStatus();
  }

  Status CheckExternalState() const override {
    return input_->CheckExternalState();
  }

  Status Get(OpKernelContext* ctx, int64 index,
             std::vector<Tensor>* out_tensors) const override {
    return input_->Get(ctx, index, out_tensors);
  }

 protected:
  Status AsGraphDefInternal(SerializationContext* ctx,
                            DatasetGraphDefBuilder* b,
                            Node** output) const override {
    Node* input_graph_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddInputDataset(ctx, input_, &input_graph_node));
    Node* buffer_size = nullptr;
    TF_RETURN_IF_ERROR(b->AddScalar(buffer_size_, &buffer_size));
    AttrValue slack_period_attr;
    b->BuildAttrValue(slack_period_, &slack_period_attr);
    AttrValue legacy_autotune_attr;
    b->BuildAttrValue(legacy_autotune_, &legacy_autotune_attr);
    AttrValue buffer_size_min_attr;
    b->BuildAttrValue(buffer_size_min_, &buffer_size_min_attr);

    TF_RETURN_IF_ERROR(
        b->AddDataset(this, {input_graph_node, buffer_size},
                      {std::make_pair(kSlackPeriod, slack_period_attr),
                       std::make_pair(kLegacyAutotune, legacy_autotune_attr),
                       std::make_pair(kBufferSizeMin, buffer_size_min_attr)},
                      output));
    return OkStatus();
  }

 private:
  class Iterator : public DatasetIterator<Dataset> {
   public:
    explicit Iterator(const Params& params)
        : DatasetIterator<Dataset>(params),
          mu_(std::make_shared<mutex>()),
          cond_var_(std::make_shared<condition_variable>()),
          buffer_size_min_(params.dataset->buffer_size_min_),
          auto_tuner_(params.dataset->buffer_size_, buffer_size_min_),
          legacy_autotune_(params.dataset->legacy_autotune_),
          // If `legacy_autotune_`, initialize the `buffer_size_` value to be 0
          // to avoid the created node to be collected as tunable nodes in the
          // autotuning optimization.
          buffer_size_(std::make_shared<model::SharedState>(
              legacy_autotune_ ? 0 : params.dataset->buffer_size_, mu_,
              cond_var_)) {
      slack_us_ = 0;
    }

    ~Iterator() override {
      CancelThreads();
      if (deregister_fn_) deregister_fn_();
    }

    bool SymbolicCheckpointCompatible() const override { return true; }

    Status Initialize(IteratorContext* ctx) override {
      mutex_lock l(*mu_);
      interleave_depth_ = ctx->interleave_depth();

      if (buffer_size_->value == model::kAutotune) {
        buffer_size_->value = buffer_size_min_;
      }
      cancellation_manager_ = std::make_unique<CancellationManager>();
      TF_RETURN_IF_ERROR(RegisterCancellationCallback(
          ctx->cancellation_manager(), [this]() { CancelThreads(); },
          &deregister_fn_));
      IteratorContext::Params params(ctx);
      params.cancellation_manager = cancellation_manager_.get();
      IteratorContext iter_ctx(params);
      TF_RETURN_IF_ERROR(dataset()->input_->MakeIterator(
          &iter_ctx, this, prefix(), &input_impl_));
      if (ctx->warm_start() && !ctx->is_restoring()) {
        TF_RETURN_IF_ERROR(EnsureThreadsStarted(ctx));
      }
      ctx->MergeCheckpoint(iter_ctx.checkpoint());
      return OkStatus();
    }

    Status GetNextInternal(IteratorContext* ctx,
                           std::vector<Tensor>* out_tensors,
                           bool* end_of_sequence) override {
      const auto& stats_aggregator = ctx->stats_aggregator();
      {
        mutex_lock l(*mu_);
        TF_RETURN_IF_ERROR(EnsureThreadsStarted(ctx));
        // Wait until the next element in the buffer has been
        // produced, or we are shutting down.
        while (buffer_.empty() && !prefetch_thread_finished_ &&
               buffer_limit() != 0) {
          if (legacy_autotune_) {
            auto_tuner_.RecordEmpty();
            buffer_size_->value = auto_tuner_.buffer_limit();
          }
          RecordStop(ctx);
          cond_var_->wait(l);
          RecordStart(ctx);
        }

        if (!buffer_.empty()) {
          return Consume(ctx, out_tensors, end_of_sequence);
        }

        if (prefetch_thread_finished_) {
          *end_of_sequence = true;
          return OkStatus();
        }

        DCHECK_EQ(buffer_limit(), 0);
      }

      mutex_lock input_l(input_mu_);
      {
        mutex_lock l(*mu_);
        if (stats_aggregator) {
          stats_aggregator->AddScalar(
              stats_utils::BufferSizeScalarName(dataset()->node_name()),
              static_cast<float>(buffer_.size()), num_elements());
          stats_aggregator->AddScalar(
              stats_utils::BufferCapacityScalarName(dataset()->node_name()),
              static_cast<float>(buffer_limit()), num_elements());
        }
        // Release mu_
      }
      return input_impl_->GetNext(ctx, out_tensors, end_of_sequence);
    }

   protected:
    std::shared_ptr<model::Node> CreateNode(
        IteratorContext* ctx, model::Node::Args args) const override {
      double buffer_size_min = buffer_size_min_;
      double buffer_size_max = std::numeric_limits<int64_t>::max();
      if (buffer_size_->value != model::kAutotune && buffer_size_->value != 0) {
        buffer_size_min = buffer_size_->value;
        buffer_size_max = buffer_size_->value;
      }
      return model::MakeAsyncKnownRatioNode(
          std::move(args),
          /*ratio=*/1,
          {model::MakeParameter(kBufferSize, buffer_size_, buffer_size_min,
                                buffer_size_max)});
    }

    Status SaveInternal(SerializationContext* ctx,
                        IteratorStateWriter* writer) override {
      if (ctx->symbolic_checkpoint()) {
        return OkStatus();
      }
      // Acquire both locks to ensure that the prefetch thread and
      // all GetNext threads are blocked.
      mutex_lock input_l(input_mu_);
      mutex_lock l(*mu_);
      TF_RETURN_IF_ERROR(SaveInput(ctx, writer, input_impl_));
      TF_RETURN_IF_ERROR(
          writer->WriteScalar(prefix(), kBufferSize, buffer_.size()));
      for (size_t i = 0; i < buffer_.size(); i++) {
        auto& buffer_element = buffer_[i];
        TF_RETURN_IF_ERROR(WriteStatus(writer, i, buffer_element.status));
        if (buffer_element.status.ok()) {
          TF_RETURN_IF_ERROR(writer->WriteScalar(
              absl::StrCat(prefix(), "::", i),
              absl::StrCat(kBuffer, kSizeSuffix), buffer_element.value.size()));
          for (size_t j = 0; j < buffer_element.value.size(); j++) {
            TF_RETURN_IF_ERROR(writer->WriteTensor(
                absl::StrCat(prefix(), "::", i),
                absl::StrCat(kBuffer, "[", j, "]"), buffer_element.value[j]));
          }
        }
      }
      return OkStatus();
    }

    Status RestoreInternal(IteratorContext* ctx,
                           IteratorStateReader* reader) override {
      mutex_lock input_l(input_mu_);
      mutex_lock l(*mu_);
      DCHECK(!prefetch_thread_);
      DCHECK(buffer_.empty());
      TF_RETURN_IF_ERROR(RestoreInput(ctx, reader, input_impl_));

      if (!ctx->symbolic_checkpoint()) {
        TF_RETURN_IF_ERROR(RestoreBuffer(ctx, reader));
      }

      if (ctx->warm_start()) {
        TF_RETURN_IF_ERROR(EnsureThreadsStarted(ctx));
      }
      cond_var_->notify_all();
      return OkStatus();
    }

    data::TraceMeMetadata GetTraceMeMetadata() const override {
      int64_t limit = -1, size = -1;
      data::TraceMeMetadata result;
      // NOTE: We only set the parallelism value if the lock can be acquired
      // right away to avoid introducing tracing overhead.
      if (mu_->try_lock()) {
        limit = buffer_limit();
        size = buffer_.size();
        if (!buffer_.empty()) {
          std::vector<std::string> shapes(buffer_.front().value.size());
          for (const auto& component : buffer_.front().value) {
            shapes.push_back(component.shape().DebugString());
          }
          result.push_back(std::make_pair("next_element_shapes",
                                          absl::StrJoin(shapes, ",")));
        }
        mu_->unlock();
      }
      result.push_back(std::make_pair(
          "buffer_limit",
          limit == -1
              ? kTraceInfoUnavailable
              : strings::Printf("%lld", static_cast<long long>(limit))));
      result.push_back(std::make_pair(
          "autotune",
          dataset()->buffer_size_ == model::kAutotune ? "true" : "false"));
      result.push_back(std::make_pair(
          "autotune_mode", legacy_autotune_ ? "legacy" : "performance"));
      if (dataset()->slack_period_ > 0) {
        result.push_back(std::make_pair(
            "slack",
            strings::Printf("%lld", static_cast<long long>(slack_us_.load()))));
      }
      result.push_back(std::make_pair(
          "interleave_depth",
          strings::Printf("%lld", static_cast<long long>(interleave_depth_))));
      return result;
    }

   private:
    // A buffer element comprises a status and (if that status is
    // OK) a vector of tensors, representing an element of the input dataset.
    struct BufferElement {
      explicit BufferElement(IteratorContext* ctx)
          : uid(tensorflow::EnvTime::NowNanos()),
            checkpoint(MemoryCheckpoint{ctx->id_registry()}) {}

      // The producer sets `status` if getting the input element fails.
      Status status;
      // The buffered data element.
      std::vector<Tensor> value;
      int64_t created_us;
      const uint64 uid;
      MemoryCheckpoint checkpoint;
    };

    Status RestoreBuffer(IteratorContext* const ctx,
                         IteratorStateReader* const reader)
        TF_EXCLUSIVE_LOCKS_REQUIRED(*mu_) {
      size_t buffer_size;
      {
        int64_t temp;
        TF_RETURN_IF_ERROR(reader->ReadScalar(prefix(), kBufferSize, &temp));
        buffer_size = static_cast<size_t>(temp);
      }
      for (size_t i = 0; i < buffer_size; i++) {
        buffer_.emplace_back(ctx);
        auto& buffer_element = buffer_.back();
        TF_RETURN_IF_ERROR(ReadStatus(reader, i, &buffer_element.status));
        if (buffer_element.status.ok()) {
          size_t value_size;
          {
            int64_t temp;
            TF_RETURN_IF_ERROR(
                reader->ReadScalar(absl::StrCat(prefix(), "::", i),
                                   absl::StrCat(kBuffer, kSizeSuffix), &temp));
            value_size = static_cast<size_t>(temp);
          }
          buffer_element.value.reserve(value_size);
          for (size_t j = 0; j < value_size; j++) {
            buffer_element.value.emplace_back();
            TF_RETURN_IF_ERROR(
                reader->ReadTensor(ctx->flr(), absl::StrCat(prefix(), "::", i),
                                   absl::StrCat(kBuffer, "[", j, "]"),
                                   &buffer_element.value.back()));
          }
        }
        RecordBufferEnqueue(ctx, buffer_element.value);
      }
      return OkStatus();
    }

    int64_t buffer_limit() const TF_EXCLUSIVE_LOCKS_REQUIRED(*mu_) {
      if (legacy_autotune_) {
        return auto_tuner_.buffer_limit();
      }
      return buffer_size_->value;
    }

    void CancelThreads() TF_LOCKS_EXCLUDED(mu_) {
      cancellation_manager_->StartCancel();
      mutex_lock l(*mu_);
      cancelled_ = true;
      cond_var_->notify_all();
    }

    Status Consume(IteratorContext* ctx, std::vector<Tensor>* out_tensors,
                   bool* end_of_sequence) TF_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
      const auto& stats_aggregator = ctx->stats_aggregator();
      if (stats_aggregator) {
        double buffer_limit_ = buffer_limit();
        stats_aggregator->AddToHistogram(
            stats_utils::BufferUtilizationHistogramName(dataset()->node_name()),
            {static_cast<float>(buffer_.size()) /
             static_cast<float>(buffer_limit_)},
            num_elements());
        stats_aggregator->AddScalar(
            stats_utils::BufferSizeScalarName(dataset()->node_name()),
            static_cast<float>(buffer_.size()), num_elements());
        stats_aggregator->AddScalar(
            stats_utils::BufferCapacityScalarName(dataset()->node_name()),
            static_cast<float>(buffer_limit_), num_elements());
      }
      // A new element is available. Forward the status from computing it, and
      // (if we successfully got an element) the output values.
      Status s = buffer_.front().status;
      if (s.ok()) {
        int64_t buffer_element_id = buffer_.front().uid;
        profiler::TraceMe traceme(
            [&] {
              return profiler::TraceMeEncode(
                  "PrefetchConsume", {{"element_id", buffer_element_id}});
            },
            profiler::kInfo);
        if (dataset()->slack_period_ > 0 &&
            (num_elements() + 1) % dataset()->slack_period_ == 0) {
          // TODO(rachelim): Consider doing something more sophisticated
          // to decide how long to sleep for; e.g. using a kalman filter.
          int64_t slack_us = EnvTime::NowMicros() - buffer_.front().created_us;
          // Every slack_period_-th element, update the most recent slack time,
          // measured by the duration between when the element is prefetched
          // and when it is consumed. We add kSleepFactor * slack_us_ to the
          // measurement because we slept for that duration before prefetching
          // the element.
          slack_us_ = kSleepFactor * slack_us_ + slack_us;
          VLOG(2) << "Setting slack_us_: " << slack_us_;
        }
        *out_tensors = std::move(buffer_.front().value);
        ctx->MergeCheckpoint(&buffer_.front().checkpoint);
        RecordBufferDequeue(ctx, *out_tensors);
      } else {
        // If status not ok, we still record the dequeue event to make sure each
        // enqueue event is paired with a dequeue event even in the presence of
        // errors.
        RecordBufferDequeue(ctx, buffer_.front().value);
      }
      if (legacy_autotune_) {
        auto_tuner_.RecordConsumption(buffer_.size());
        buffer_size_->value = auto_tuner_.buffer_limit();
      }
      buffer_.pop_front();
      *end_of_sequence = false;

      // Wake the prefetch thread, in case it has been waiting for space
      // in the buffer. Also wake up threads from other calls to GetNext.
      //
      // TODO(mrry): Consider using different condition variables for
      // GetNext and Prefetch.
      cond_var_->notify_all();
      return s;
    }

    Status EnsureThreadsStarted(IteratorContext* ctx)
        TF_EXCLUSIVE_LOCKS_REQUIRED(*mu_) {
      if (!prefetch_thread_) {
        std::shared_ptr<IteratorContext> new_ctx =
            std::make_shared<IteratorContext>(*ctx);
        prefetch_thread_ = ctx->StartThread(
            "tf_data_prefetch", [this, new_ctx]() { PrefetchThread(new_ctx); });
      }
      return OkStatus();
    }

    // Prefetches elements of the input, storing results in an internal buffer.
    //
    // It owns the iterator context passed to it.
    void PrefetchThread(const std::shared_ptr<IteratorContext>& ctx) {
      RecordStart(ctx.get());
      auto cleanup = gtl::MakeCleanup([this, ctx] { RecordStop(ctx.get()); });
      // Keep track of where we are in an iteration "burst"
      int num_produced = 0;
      while (true) {
        // 1. Wait for a slot in the buffer.
        {
          mutex_lock l(*mu_);
          while (!cancelled_ && buffer_.size() >= buffer_limit()) {
            RecordStop(ctx.get());
            cond_var_->wait(l);
            RecordStart(ctx.get());
          }

          if (cancelled_) {
            prefetch_thread_finished_ = true;
            cond_var_->notify_all();
            return;
          }
        }

        if (dataset()->slack_period_ > 0 &&
            num_produced % dataset()->slack_period_ == 0) {
          // For the first element in the "burst", sleep for a bit if there is
          // slack.
          VLOG(2) << "Sleeping for: " << slack_us_ * kSleepFactor;
          ctx->env()->SleepForMicroseconds(slack_us_ * kSleepFactor);
        }

        // 2. Read the next element.
        // Acquire the input mutex since we will be reading an element from the
        // input iterator. Note that we do not wish to release this mutex till
        // we have added the fetched element to the `buffer_` else there will be
        // local state that may be missed by SaveInternal.
        mutex_lock input_l(input_mu_);
        bool end_of_sequence = false;
        BufferElement buffer_element(ctx.get());
        {
          profiler::TraceMe traceme(
              [&] {
                return profiler::TraceMeEncode(
                    "PrefetchProduce", {{"element_id", buffer_element.uid}});
              },
              profiler::kInfo);
          buffer_element.status = input_impl_->GetNext(
              ctx.get(), &buffer_element.value, &end_of_sequence);
          buffer_element.checkpoint.Merge(ctx->checkpoint());
        }
        if (buffer_element.status.ok() && end_of_sequence) {
          mutex_lock l(*mu_);
          prefetch_thread_finished_ = true;
          cond_var_->notify_all();
          return;
        }

        // 3. Signal that the element has been produced.
        {
          mutex_lock l(*mu_);
          RecordBufferEnqueue(ctx.get(), buffer_element.value);
          buffer_element.created_us = EnvTime::NowMicros();
          buffer_.push_back(std::move(buffer_element));
          cond_var_->notify_all();
        }
        ++num_produced;
      }
    }

    Status WriteStatus(IteratorStateWriter* writer, size_t index,
                       const Status& status) TF_EXCLUSIVE_LOCKS_REQUIRED(*mu_) {
      TF_RETURN_IF_ERROR(
          writer->WriteScalar(absl::StrCat(prefix(), "::", index), CodeKey(),
                              static_cast<int64_t>(status.code())));
      if (!status.ok()) {
        TF_RETURN_IF_ERROR(writer->WriteScalar(
            absl::StrCat(prefix(), "::", index), ErrorMessageKey(),
            std::string(status.message())));
      }
      return OkStatus();
    }

    Status ReadStatus(IteratorStateReader* reader, size_t index, Status* status)
        TF_EXCLUSIVE_LOCKS_REQUIRED(*mu_) {
      int64_t code_int;
      TF_RETURN_IF_ERROR(reader->ReadScalar(absl::StrCat(prefix(), "::", index),
                                            CodeKey(), &code_int));
      absl::StatusCode code = static_cast<absl::StatusCode>(code_int);

      if (code != absl::StatusCode::kOk) {
        tstring error_message;
        TF_RETURN_IF_ERROR(
            reader->ReadScalar(absl::StrCat(prefix(), "::", index),
                               ErrorMessageKey(), &error_message));
        *status = Status(code, error_message);
      } else {
        *status = OkStatus();
      }
      return OkStatus();
    }

    string CodeKey() { return absl::StrCat(kStatus, kCodeSuffix); }

    string ErrorMessageKey() {
      return absl::StrCat(kStatus, kErrorMessageSuffix);
    }

    // This mutex is used to ensure exclusivity between multiple threads
    // reading/writing this iterator's local state.
    //
    // NOTE: We should never call GetNext on the input while holding this mutex.
    const std::shared_ptr<mutex> mu_;
    // This mutex is used to ensure exclusivity between multiple threads
    // accessing the input iterator. We keep this separate from `mu_` to allow
    // prefetching to run in parallel with GetNext calls.
    mutex input_mu_ TF_ACQUIRED_BEFORE(*mu_);
    // Controls cancellation of `input_impl_`. Must be ordered before
    // `input_impl_` so that `input_impl_` is destroyed first.
    std::unique_ptr<CancellationManager> cancellation_manager_;
    std::unique_ptr<IteratorBase> input_impl_ TF_GUARDED_BY(input_mu_);
    const std::shared_ptr<condition_variable> cond_var_;
    const int64_t buffer_size_min_;
    PrefetchAutotuner auto_tuner_ TF_GUARDED_BY(*mu_);
    std::deque<BufferElement> buffer_ TF_GUARDED_BY(*mu_);
    bool cancelled_ TF_GUARDED_BY(*mu_) = false;
    bool prefetch_thread_finished_ TF_GUARDED_BY(*mu_) = false;
    const bool legacy_autotune_;

    std::atomic<int64_t> slack_us_;

    // If legacy_autotune_ is false, identifies the maximum size of the buffer.
    const std::shared_ptr<model::SharedState> buffer_size_;

    // Method for deregistering the cancellation callback.
    std::function<void()> deregister_fn_;

    // Records the number of ParallelInterleave operations in the path from the
    // root node to this node (not including this node) in the input pipeline
    // tree. We record the interleave depth so that it can be included in the
    // trace metadata.
    int64 interleave_depth_ = -1;
    std::unique_ptr<Thread> prefetch_thread_ TF_GUARDED_BY(*mu_);
  };

  const DatasetBase* const input_;
  const int64_t buffer_size_;

  // If non-zero, determines the period between injecting "slack" into the
  // execution.
  const int64_t slack_period_;

  // Determines whether legacy autotuning should be used.
  const bool legacy_autotune_ = true;

  // If autotune is enabled, determines the minimal value of `buffer_size`
  // parameter.
  const int64_t buffer_size_min_ = 0;

  TraceMeMetadata traceme_metadata_;
};

PrefetchDatasetOp::PrefetchDatasetOp(OpKernelConstruction* ctx)
    : UnaryDatasetOpKernel(ctx) {
  if (ctx->HasAttr(kSlackPeriod)) {
    OP_REQUIRES_OK(ctx, ctx->GetAttr(kSlackPeriod, &slack_period_));
  }
  if (ctx->HasAttr(kLegacyAutotune)) {
    OP_REQUIRES_OK(ctx, ctx->GetAttr(kLegacyAutotune, &legacy_autotune_));
  }
  if (ctx->HasAttr(kBufferSizeMin)) {
    OP_REQUIRES_OK(ctx, ctx->GetAttr(kBufferSizeMin, &buffer_size_min_));
  }
  if (GetExperiments().contains("autotune_buffer_optimization")) {
    legacy_autotune_ = false;
    buffer_size_min_ = std::max(static_cast<int64_t>(1), buffer_size_min_);
  }
}

void PrefetchDatasetOp::MakeDataset(OpKernelContext* ctx, DatasetBase* input,
                                    DatasetBase** output) {
  int64_t buffer_size = 0;
  OP_REQUIRES_OK(ctx,
                 ParseScalarArgument<int64_t>(ctx, kBufferSize, &buffer_size));
  OP_REQUIRES(ctx, buffer_size >= 0 || buffer_size == model::kAutotune,
              errors::InvalidArgument("buffer_size must be >= 0 or set "
                                      "buffer_size to be ",
                                      model::kAutotune, " for auto-tuning"));

  if (buffer_size == model::kAutotune) {
    metrics::RecordTFDataAutotune(kDatasetType);
  }

  *output = new Dataset(ctx, input, buffer_size, slack_period_,
                        legacy_autotune_, buffer_size_min_);
}

namespace {
REGISTER_KERNEL_BUILDER(Name("PrefetchDataset").Device(DEVICE_CPU).Priority(2),
                        PrefetchDatasetOp);
REGISTER_KERNEL_BUILDER(Name("PrefetchDataset")
                            .Device(DEVICE_GPU)
                            .HostMemory("buffer_size")
                            .HostMemory("input_dataset")
                            .HostMemory("handle")
                            .Priority(1),
                        PrefetchDatasetOp);
}  // namespace

}  // namespace data
}  // namespace tensorflow
